Field of the Disclosure
The present disclosure relates to a focus detection unit, an image capturing apparatus including the focus detection unit, and a focus detection method.
Description of the Related Art
Conventionally, in an image capturing apparatus, an autofocus function (hereinbelow referred to as an AF) is known as a function to perform focus adjustment. In this AF technique, various methods such as a phase difference method and a contrast detection method are proposed.
For example, in the AF function in the contrast detection method, a value generated based on contrast of a high frequency component extracted from an image signal is used as an AF evaluation value. In a state in which an image is defocused, the contrast of the high frequency component (that is, the AF evaluation value) is low. As the image is further focused, the contrast of the high frequency component is higher. Thus, by adjusting a focus position so that the AF evaluation value may be the maximum, the image can be focused. This is how the AF function in the contrast detection method works.
Besides such a basic AF control method, Japanese Patent Publication No. 4641494 discloses a technique enabling improvement of focusing accuracy by generating a plurality of AF evaluation values in different frequency bands and, at the time of in-focus, using a signal of a higher frequency component than a frequency band normally used.
An auto exposure control function (AE) is generally used, in which, when a subject is dark in an image capturing environment, a gain is multiplied to an image signal to compensate for the darkness to keep an appropriate exposure level.
When the process of multiplying a gain when the subject is dark is performed in this manner, a noise is carried on the image signal, and a noise signal is thus superimposed on the high frequency component used for the AF evaluation value. This noise has a greater influence on the image signal as the gain to be multiplied is higher (high gain). Also, the noise signal generated by gain multiplication has a greater influence on the high frequency component than on the low frequency component.